This invention has been publicly disclosed in the peer reviewed journal article: Hull E L, R. H. Pehl, J. R. Lathrop, B. S. Suttle, “Yttrium hole-barrier contacts for germanium semiconductor detectors.” Nucl. Instr. and Meth. A 626-627 (2011) p. 39-42 (2011), having an online publication date of Oct. 14, 2010.
Germanium semiconductor single particle radiation detectors require both electron-barrier and hole-barrier contacts to provide full depletion and sufficient electric field for good charge-carrier collection while blocking the flow of significant leakage current. In addition, the contacts must provide a sufficiently conductive electrical connection to the germanium to avoid series noise problems. The negatively biased electron-barrier contact can be fabricated using several well developed technologies. Electron-barrier contacts can be fabricated by deposition of thin (˜1000 Å) metal layers directly onto the crystalline germanium surface. Gold, nickel, chromium, platinum, and palladium have been demonstrated to form good Schottky electron-barrier contacts on germanium semiconductor radiation detectors. The most comprehensive publication describing metal electron-barrier contacts is H. L. Maim, “Properties of Metal Surface Barriers on High Purity Germanium,” IEEE Trans. Nucl. Sci., NS-22, p. 140, (1975). Most germanium detector manufacturing now relies on thin boron implanted p+ contacts to provide the electron-barrier contact. All these electron-barrier contacts are sufficiently thin to allow segmentation into arbitrary contact geometries and provide thin dead layer entrance windows on the live detector volume for minimal charged particle energy loss and photon attenuation.
Historically, the use of germanium semiconductor radiation detectors has been adversely affected by the lack of a convenient thin hole-barrier contact to serve opposite these thin electron-barrier contacts. The industry relies upon thick, as thick as 1 mm, lithium diffused n+ hole-barrier contacts as the standard contacts. Although lithium contacts can be made less thick, they cannot approach the 1000 Å thickness level required for a truly thin particle entrance window. Although extremely rugged, reliance upon thick lithium diffused contacts has greatly limited the use of germanium detectors as transmission detectors in charged-particle telescopes for nuclear physics experiments. In addition, lithium diffused contacts prohibit the transmission of low energy photons for low energy photon spectroscopy. The thin electron-barrier contact must always be used as the particle entrance window. Lithium diffused contacts can be coarsely segmented by grinding through the lithium diffused layer and/or by the use of relatively wide gap features between segments. However, the inherent thickness of the lithium diffused layer presents a significant limitation to the segmentation feature sizes possible. A thin hole-barrier contact that permits low energy photon transmission and that can be finely segmented in a convenient manner would be a tremendous improvement in germanium-detector technology.
The search for a thin hole-barrier contact on germanium detectors has continued since the 1970s. Phosphorus implanted n+ contacts were successfully implemented in long standing nuclear physics array and telescope programs as described in G. S. Hubbard, E. E. Haller, W. L. Hansen, “Ion Implanted N-type Contact for High-Purity Germanium Radiation Detectors,” IEEE Trans. Nucl. Sci. NS-24 No. 1, p. 161, (1977). However, fabrication of the phosphorus implanted n+ contact is an extremely involved process requiring high temperature annealing steps that can significantly harm the charge collection properties of the germanium crystal. Despite great effort, phosphorus contacts have not consistently supported the high electric fields needed for optimum charge collection. This is particularly crucial in situations where radiation damage is a concern. Few, if any, phosphorus implanted n+ contacts are manufactured on germanium detectors at this time. Most detector makers have turned to amorphous germanium and amorphous silicon contacts for thin electron-barrier and/or hole-barrier contacts requiring segmentation. Amorphous germanium contacts were first described in W. L. Hansen and E. E. Haller, “Amorphous germanium as an electron or hole blocking contact on high-purity germanium detectors,” IEEE Trans. Nucl. Sci., NS-24, No. 1, p. 61, (1977) and later in P. N. Luke, C. P. Cork, N. W. Madden, C. S. Rossington, M. F. Wesela, “Amorphous Ge Bipolar Blocking Contacts on Ge Detectors,” IEEE Trans. Nucl. Sci. 39 No. 4, p. 590 (1992). Unfortunately, amorphous semiconductor contacts are not as stable or repeatable as desired. Amorphous germanium contacts do not always form sufficiently high charge injection barriers to prevent significant leakage current at higher temperatures (˜95 K). However, compared to phosphorus implantation or lithium segmentation, amorphous germanium and amorphous silicon contacts require far less fabrication complexity and support reasonably high electric fields on a sufficiently consistent basis to be useable at lower detector operating temperatures (˜85 K). Although not ideal, amorphous germanium and amorphous silicon contacts have provided the most versatile thin and segmented contacts for germanium detectors in recent years.